Vibration sensors play a role in many areas of technology. For example, systems for active vibration isolation are based on a vibration measurement, which only makes it possible to actively damp unwanted vibrations. A further field of application for vibration sensors is ascertaining the imbalance of rotating shafts, e.g., in machine tools.
One use of vibration sensors is described in European Published Patent Application No. 2 719 499, according to which, the imbalance of a rotating shaft of a machine tool is monitored with the aid of a vibration sensor that is disposed on the stationary part of a machining center having rotating shafts. Other placements of the vibration sensor, for instance, directly on a rotary table, are also discussed in the introduction thereof.
In the technical field of active vibration isolation, for which a good overview is provided in European Published Patent Application No. 2 075 484, what are referred to as geophones are often used in conventional methods. Basically, they are sensors in which a flexibly supported magnet as a test mass is caused to vibrate by external excitation. In response, coils in the vicinity of the magnet output a voltage signal proportional to she speed. Such inductive sensors have the disadvantage, however, that the signal-to-noise ratio at low vibrational frequencies becomes small, because low frequency means a slow movement of the magnet relative to the coil, and therefore only a small induced voltage. Geophones for she exact detection of vibrations below 4 Hz are obtainable only with great difficulty. However, the natural frequencies of platforms on which moving devices are arranged are also on this order of magnitude, and to the greatest extent possible, the platforms should not be stimulated to vibrate, either due to external influences such as a vibrating foundation, or by internal excitation due to the moving device itself. Systems for active vibration isolation therefore measure the vibrations of the platform and attenuate them actively, e.g., with the aid of actuators which act between the platform and the foundation.
European Published Patent application No. 2 075 484, for example, describes, instead of using inductive speed sensors susceptible to noise, using position-measuring devices which are able to detect the movement of a test mass even in the case of very slow movements, without additional noise occurring in the process. A position-measuring device is also able to detect the excursion of a test mass in the case of very slow movements at any time (even during standstill), so that low frequencies do not affect the signal-to-noise ratio detrimentally.
U.S. Pat. No. 6,473,187 describes a vibration sensor having a mass block which is supported with the aid of a leaf spring in a manner allowing movement, relative to a frame in a measuring direction, a displacement of the mass block relative to the frame being detectable by a position-measuring device. Disposed on the mass block and on the frame are fingers intermeshing in comb-like fashion, which together result in an evenly-spaced optical diffraction grating at which light from a light source is reflected into different orders of diffraction and is then detected by a detector. In response to an excursion of the proof mass, the effective period of the grating, and therefore the distribution of the light into the various orders of diffraction, changes. Consequently, it is possible to infer the excursion of the proof mass from the detector signals. However, the finger-like and self-supporting structures acting as an optical diffraction grating are not easy to produce, and the grating periods of less than one micrometer customary especially for a highly accurate position measurement cannot be achieved with such structures.
Commercially available position-measuring devices, which are described in detail, for example, in the reference book “Digital Linear and Angular Metrology,” Moderne Industrie Publishing House, Landsberg/Lech, 1998, have a measuring standard with fine periodic structures that are scanned by a scanning head moved relative to the measuring standard. For example, the periodic structures are able to modulate the reflectivity, which may be scanned with the aid of light. In that context, measuring standards having graduation periods of less than one micrometer are already being used. By the use of interferential scanning using monochromatic light and further subdivision of the periodic detector signals by interpolation, it is possible to determine position changes in the nanometer range.